The first goal of the experiments described in this application is to delineate the extent to which the collicular visual and somatosensory representations follow interdependent developmental programs. The second aim is to determine whether or not the laminar and topographic organizations of tectal sensory inputs affect the development of efferent projections from this nucleus. To accomplish our first goal, we will examine the normal ontogeny of the visual and somatosensory representations with extracellular single unit recording methods. We will also experimentally alter either the visual or somatosensory map by means of neonatal surgery and determine whether or not this produces a concomitant change in the collicular representation we have not directly manipulated. Extracellular single unit recording and receptive field mapping techniques will again be employed to assess the consequences of our experimental intervention in the normal development of these animals. We will also determine the effects of altered sensory organization upon the ontogeny of three tectofugal pathways which we have described in detail in normal adult hamsters (see below for references). These are the tectospinal projection, the intercollicular pathway, and the collicular projection to the lateral posterior nucleus. These projections arise from neurons which have clearly different laminar distributions, and functional and morphological characteristics. We will use multiple retrograde axonal tracing methods to determine: (a) whether or not these distinct projections are achieved by the elimination of axon collaterals in neurons which project to multiple targets for a limited period during infancy, and (b) whether or not alterations in the laminar or topographic organization of sensory inputs change the distribution of neurons contributing axons to a given tectofugal pathway.